The progress of lost core technology for fabricating complex plastic parts with a hollow curved internal space has faced the problem of dimensional stability and consequent reproducibility in mass production. Such dimensional nonuniformity is the result of weak strength at a higher molding temperature for manufacturing thermoset or thermoplastic parts. As a core material, tin-based alloys were employed as stated in prior art [1. Automotive Engineer v.12 n.1 Feb/Mar (1987) p 38; 2. British Pat. GB 2165860 A (UK) (1985); 3. Automobil Tech. Z. v.89 n.3 Mar (1987) p 139]. The weakness problem of such monolithic alloys has been solved by applying the concept of composites technology and thus fiber reinforced metal matrix composites have been invented as described in prior art by the present inventors [4. U.S. Pat. application Ser. No. 07/166,060 filed Mar. 9, 1988; 5. U.S. Pat. application filed Apr. 27, 1988 "Development of Fusible Alloy Composites"]. It is now possible to produce cores with a higher strength than monolithic alloys while maintaining the melting point of new core materials in the same range of conventional monolithic alloys. The cost of producing such fibrous metal matrix composites was about the same as conventional alloys. It will be economically desirable to produce acceptable core materials at a cheaper cost than traditional alloys. It is the goal of this invention to produce such core materials.